Prior clutch or brake devices have used various types of rollers, sprags, or cam elements for engaging and releasing torque transmission between input and output members. Such locking elements have generally taken the form of cylindrical rollers, spherical balls, or cam shapes of various designs. The locking elements in conventional clutch or brake devices have a limit to their torque capacity due to their design and the loading dynamics (two-dimensional in nature). The wear on these conventional locking elements becomes excessive (due to high contact stresses) as the load increases. This reduces the life cycle and performance of the unit and may also result in the elements going "over-center" as the wear progress.
In an over-center situation the locking elements could be placed in a state where they are unable to transmit any load from the input to the output member (free wheeling in both directions), or it is also possible that they could become jammed between the inner and outer races thus permanently coupling the input to the output. In an effort to reduce stress on mechanical members, conventional over-running clutches are forced to grow both in mass and volume, thus limiting their use to non-critical mass/volume applications. Particle generation due to excessive wear has also, in the past, prevented the use of conventional clutch or brake devices in cleanroom and aerospace applications.
Recent developments have used locking elements or sprags between the inner and outer races of a clutch or brake which have a three-dimensional shape in the direction of the Z (rotational) axis. Such 3D sprags provide performance and service life characteristics that are superior to conventional locking elements which have only two-dimensional (2D) geometries (i.e., contact surfaces shaped only relative to the plane normal to the Z axis, but not shaped along the Z axis), because effective locking is achieved at sprag angles that are larger than those used in 2D shapes (e.g., 6 degrees). The 3D geometry in the Z-axis direction, and the subsequent increase in sprag locking angle (e.g., 12 degrees), allow the clutch or brake device to withstand higher loads with lower wear.
Although 3D sprags provide a remarkable advantage over 2D sprags in load capacity and housing weights, their loading surfaces could be further optimized. The 3D sprags interface with the convex/concave races of the clutch or brake device at essentially point contacts (in cross-section). Such point contacts result in contact (Hertzian) stresses which, although low compared to 2D sprags, are nevertheless high enough in some circumstances to warrant improvement. Also, it is desirable to improve the design of 3D sprags to provide other advantages, such as longer service life, better wear characteristics, reliability of operation, etc.